Hemoglobin molecule is a prototype of an allosteric protein whose function is governed by its conformation which in turn is regulated by a large number of small molecules such as organic phosphates, CO2, C1-, etc. (allosteric effectors). This work deals with the study of the influence of conformational changes on enzymatic methemoglobin reduction as well as with several aspects of oxidation. Methemoglobin reductase was purified by conventional methods and methemoglobin reduction was studied. It was shown that the reduction rate is conformational dependent, T conformation being the preferred substrate. For this reason the presence of inositol hexaphosphate (IHP) significantly accelerate the reaction rate. This effect is specific and cannot be produced by NaCl. Besides the beta subunits have 4-6 fold higher reduction rate. The study of oxidation of hemoglobin by nitrite has revealed that although the blockage of beta-93-SH group accelerates dramatically the rate of oxidation of deoxyhemoglobin, the addition of IHP has the same inhibitory effect as it does on native hemoglobin. Besides higher concentration of IHP inhibits as well the oxidation of myoglobin by nitrite. It is concluded that: 1) the organic phosphates play an important role in methemoglobin reduction; and 2) although most of the IHP inhibitory effect on the oxidation of hemoglobin by nitrites is due to the shift of the molecular conformation towards T, not all of its effect can be explained in this fashion. IHP must bring about some other subtle structural changes which affect the oxidation rate.